Field of the Invention
This invention relates to a semiconductor device and, in particular, to a semiconductor device such as a light emitting device or the like using II-VI compound semiconductors.
Description of the Prior Art
Recently, in order to improve the recording density of optical discs and the resolution of laser printers, semiconductor lasers capable of emitting light having short wavelength are increasingly desired, and there are active researches to realize them.
The present Applicant made vigorous researches to meet the demand and proposed a semiconductor laser capable of emitting blue or green light which uses a sort of II-VI compound semiconductors, namely, ZnMgSSe compound semiconductor, as the material for its cladding layers. In this semiconductor laser, a laser structure including an n-type ZnMgSSe cladding layer, an active layer and a p-type ZnMgSSe cladding layer (not shown) is provided on an n-type GaAs substrate, and a p-type ZnSe contact layer is provided on the p-type ZnMgSSe cladding layer. On the p-type ZnSe contact layer is provided a p-side electrode, and on the back surface of the n-type GaAs substrate is provided an n-side electrode.
With this structure, the p-side electrode has a high contact resistance with respect to the p-type ZnSe contact layer, and it is difficult to obtain a good ohmic contact. Major reasons thereof are that the carrier concentration obtained by doping p-type impurities into ZnSe is as low as the order of .about.10.sup.17 cm.sup.-3 at maximum and that it is difficult to obtain a good ohmic contact with p-type ZnSe.
In addition, there is a band discontinuity in the valence band as large as 0.5 eV at the junction interface between p-type ZnSe (energy gap: 2.8 eV) and p-type ZnTe (energy gap: 2.3 eV). The valence band of p-type ZnSe is curved downward toward p-type ZnTe (see FIG. 4), and the changes in the valence band bulging downward behave as a potential barrier against holes injected from the p-side electrode to the p-type ZnSe/p-type ZnTe junction.